It's a frightening reality: Americans now live in a country that could be facing a terrorist threat worse than al Qaeda: the Islamic State.

U.S. intelligence officials warn that ISIS has now gained momentum that will be difficult to halt in Iraq and Syria. They say the Islamic terrorists intend to prove their power far beyond the Middle East.

Intelligence warns the Islamic State may attempt a major terrorist strike in Europe or the United States to assert itself as the true leader of the international jihadist movement.

ISIS is using social media to advance its in-your-face propaganda.

Recently, a photo was tweeted of a hand holding an ISIS flag right in front of the White House. It was pictured with a hand-written warning attached:

"A message from ISIS to U.S. - We are in your state. We are in your cities. We are in your streets," the Aug. 9 note read.

Another photo was shown in the same picture. It featured a handwritten note in Arabic held in front of a building with two American flags at an unclear location.

Translated, the note dated June 20, 2014 says: "Soldiers of the Islamic State of Iraq and Syria will pass from here soon," followed by a verse from the Koran.

The Secret Service said it's investigating the matter.

"We have an intelligence division whose mission is to assess information that we receive every day for dangerousness or potential threat level. We are aware of the image and will take the necessary and appropriate follow up steps," Secret Service spokesman Ed Donovan told ABC News.

Meanwhile, a New Jersey man said he meant no offense when he flew a flag that looks like the ISIS flag in front of his home in New Jersey.

Mark Dunaway, from Winslow Place, New Jersey, said he was expressing his Muslim religion by flying the flag every year during Ramadan and every Friday.

"Every Muslim uses that black flag," NJ.com quoted Dunaway. "You'll find it in any mosque in world. I am an American citizen and I love my country, but I am also a Muslim and I use that flag to say I'm a Muslim."

But a spokesman for the Council on American-Islamic Relations told ABC News Dunaway's flag isn't the Islamic flag.

The scare comes as President Barack Obama announced there will be no need for the United States to rescue stranded Yazidis from Iraq's Mount Sinjar, saying U.S. airstrikes allowed most to escape.